friendfeed primarily takes multiple rss feeds and presents them as a single list. the funny thing is most RSS readers already do this. Taking as an example, Veronica Belmont and Chris Messina, i can get their 'activity stream' by subscribing to their individual feeds as demonstrated in google reader:
Notice the 'blogs' folder is selected and that displays the contents all the rss feeds as a single page.
friendfeed makes it easy to manage groups of urls. each group describes one person. i can spot a friend of a friend and add them as a friend. with one click i get their dozen rss feeds blended into my daily reading stream.
my friendfeed is available as a feed of its own. so i could stick that into my rss reader and let friendfeed do the merging and managing. that is cool for sure but i don't like being dependent on a single service provider. i mostly like the idea of caching and merging those RSS feeds on my own box, the way an rss reader application does.
friendfeed gets huge props for 'imaginary friends'. other services try to keep you in their silo by saying you can only follow a friend if that friend has signed up for our service (readr.com seems to work this way). in friendfeed i can create a profile for another person and add URLs to that profile without that person having to signup for friendfeed. i'm saddled with managing changes to that profile since its really my profile, but describes someone else.
instant messaging programs already have a concept of a friends list. they're a local application and they have a slick UI. i use pidgin in ubuntu and vista. i looked at adding the last twitter update to each friend listed in pidgin. the problem is pidgin doesnt support changing the buddy list view with a plugin (according to a guy in #pidgin), plus it doesnt support a conceptual user that is represented by an AIM account, a gmail account ,etc.. I would need to use that more abstract user concept to thrown in the twitter feed. Trillium for the PC has this concept.